Laser Direct Writing of Tree-Shaped Hierarchical Cones on aSuperhydrophobic Film for High-Efficiency Water CollectionMeng Wang,†,‡ Qian Liu,§ Haoran Zhang,§ Chuang Wang,§ Lei Wang,§ Bingxi Xiang,†,‡ Yongtao Fan,∥

Chuan Fei Guo,*,⊥ and Shuangchen Ruan*,†,‡

†Shenzhen Key Laboratory of Laser Engineering, College of Optoelectronic Engineering and ‡Key Laboratory of OptoelectronicDevices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, ShenzhenUniversity, Shenzhen 518060, People’s Republic of China⊥Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, People’sRepublic of China§National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China∥Shanghai Institute of Optics and Fine Mechanics, Shanghai 201800, People’s Republic of China

*S Supporting Information

ABSTRACT: Directional water collection has stimulated a greatdeal of interest because of its potential applications in the field ofmicrofluidics, liquid transportation, fog harvesting, and so forth.There have been some bio or bioinspired structures for directionalwater collection, from one-dimensional spider silk to two-dimensional star-like patterns to three-dimensional Nepenthesalata. Here we present a simple way for the accurate design andhighly controllable driving of tiny droplets: by laser direct writingof hierarchical patterns with modified wettability and desiredgeometry on a superhydrophobic film, the patterned film canprecisely and directionally drive tiny water droplets anddramatically improve the efficiency of water collection with a factor of ∼36 compared with the original superhydrophobicfilm. Such a patterned film might be an ideal platform for water collection from humid air and for planar microfluidics withouttunnels.

KEYWORDS: laser direct writing, water collection, superhydrophobic, hierarchical patterns, microfluidics

■ INTRODUCTION

Water collection and shortage is a big challenge, which isespecially critical in dry and water-laden areas, such as in thedesert or in the seaside. In nature, some creatures includingdesert beetles,1 spider silk,2 cacti,3 and pitcher plants4 havedeveloped the amazing ability to collect water with some specialmicro- and nanostructures on their surfaces. Some currenttechnologies of water collection were inspired by such naturalstructures.5−10 For example, unprecedented droplet growth andtransport have been achieved by designing structuresmimicking Namib desert beetles, cacti, and pitcher plants.Bioinspired artificial spider silks with controllable spindle-knotgeometry have been successfully fabricated and shownoutstanding water-collecting ability.11,12 Water could betransported from the inner side to the outer side on thesuperhydrophilic artificial Nepenthes alata surface.4,13 Thesebioinspired structures are mostly one dimensional or threedimensional, which are relatively difficult to design andfabricate. By contrast, a two-dimensional (2D) surface isquite compatible with the modern micro/nanofabricationtechniques and very commonly used in industry. Bai et al.also successfully made 2D patterns with cones (such as stars)

for water collection, exhibiting a significant enhancement ofefficiency of ∼5 times.14 However, the existing methods exhibiteither limited resolution or limited capacity of large areafabrication. In addition, it is difficult to spin coat a layer ofphotoresist on a surperhydrophobic surface such that somelithographic method may not be used for making patterns on asuperhydrophobic film. Hierarchical structures are found to bemore efficient for water collection than simple patterns;3 it istherefore urgent to develop a method that can generate high-resolution and large area hierarchical patterns with controllablesurface wettability for highly efficient water collection.Laser direct writing (LDW), a laser-beam based technique,

can directly make complicated patterns with a high resolution(a few hundred nanometers), high writing speed, large area, andhigh flexibility on a 2D surface.15−19 This technique can makepatterns not only on photoresists20,21 but also on metal22 andceramic films,23 allowing for micronanofabrication or surfacemodification of a variety of materials. Especially for the

Received: June 7, 2017Accepted: August 14, 2017Published: August 14, 2017

Research Article

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© 2017 American Chemical Society 29248 DOI: 10.1021/acsami.7b08116ACS Appl. Mater. Interfaces 2017, 9, 29248−29254

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patterning on metals and ceramics, LDW can modify thesurface properties or morphology by a simple one-step process.Here, we report on that by using LDW to write tree-shapedhierarchical cones on a superhydrophobic TiO2 film with asurface energy gradient and Laplace pressure gradient; thepatterned film can precisely and continually drive tiny waterdroplets toward the target location in the horizontal planewithout the help of gravity. As a result, this type of surface ismore highly efficient with a factor of ∼36 in directional watercollection than a film without the pattern. Moreover, this studyprovides insight into the design of a novel patterned film withcomplex wettability and “planar microfluidic”. It can be used toenhance the efficiency in directional water collection ordroplets manipulation and might be extended to more diverseapplications. This work further confirms that the geometry is acritical factor for water collection design and other possibleapplications in surface science.

■ EXPERIMENTAL SECTIONLaser Direct Writing of Porous TiO2 Films. Ti films with a

thickness of 100 nm were deposited on silicon substrates byradiofrequency magnetron sputtering (ULVAC ACS400-C4) with apower of 50 W and working pressure of 0.57 Pa for 3000 s and thenimmersed in aerated 10 M NaOH at 60 °C for 30 min to prepare theporous TiO2 on the surface. The porous TiO2 films were modifiedwith 1H,1H,2H,2H-perfluorodecyltriethoxysilane through chemicalvapor deposition (CVD) at 120 °C for 4 h to achieve super-hydrophobicity. A laser direct writing system (HWN laser directwriting system-1500) was used for writing patterns on the super-hydrophobic surface. The laser writer applied a laser with a wavelengthof 405 nm, a laser spot size of ∼300 nm, a pulse duration of 2000 ns, apoint spacing of 100 nm, a repetition rate of 500 kHz, and an energydensity of 56.6 J/cm2. The samples were raster scanned at a linearspeed of 100 mm s−1 with a parallel line density of 2500 mm−1.

Characterization. The morphology of the superhydrophobicporous TiO2 surface was observed by field emission scanning electronmicroscopy (FESEM, Hitachi S-4800), a surface profiler (VeecoDektak 150, tip radius 12.5 μm), and a laser scanning confocalmicroscope (LSCM, Olympus, LEXT-OLS 4000). The composition ofthe films was analyzed by X-ray photoelectron spectroscopy (XPS).

Figure 1. (a−c) Schematic illustration for the fabrication process of bioinspired surfaces with anisotropic micropatterns. (a) Superhydrophobicporous TiO2 surface showing nonwetting property to fog droplets. (b) Laser writing of a hierarchical cone structures on the TiO2 film. (c)Directional driving of tiny droplets with a smart surface. (d−e) SEM images of the superhydrophobic porous titanium oxide surface (d) before and(e) after LDW process. (f) Boundary between laser-processed and unscanned areas. (g) 3D surface profile of stripes fabricated by LDW. (h) Surfaceprofile along the y direction of the surface in panel. (i) XPS spectra of the laser-treated and untreated area.

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The water contact angle (CA) was measured at ambient temperatureusing an OCA 20 instrument (Data-physics, Germany). Deionizedwater (Millipore, 18 MΩ cm) was employed as the source for thecontact angle measurement. The CA values were the average of fivedrops at different locations. The processes of water collection wasobserved by optical microscopy and recorded using a charge-coupleddevice camera (CCD). A fog flow generated by an ultrasonichumidifier was used to examine the water collection properties.

■ RESULTS AND DISCUSSION

Controllable Wettability of a TiO2 Surface Using LDW.TiO2 films with micropatterns were fabricated following theprocedures in Figure 1. First, a porous TiO2 surface wasfabricated with a hydrothermal method by immersing a metalTi film in sodium hydroxide solution followed by depositing alayer of 1H,1H,2H,2H-perfluorodecyltriethoxysilane, showingnonwetting property to fog droplets (Figure 1a). After that thesuperhydrophobic TiO2 surface was patterned by laser writingat a high resolution of 300 nm. The laser beam removes thesurface structures, making the film smoother and relativelywettable (Figure 1b). On the surfaces with laser-scannedpatterns, water droplets were found to directionally aggregatetoward more wettable regions (Figure 1c). The laser scanned

area was found to have a significant change in surfacemorphology, as shown in Figure 1d−f. Figure 1d shows anas-made superhydrophobic titanium oxide surface with a highlyporous surface.24 Such a film exhibits superhydrophobicproperty with a CA as high as 160° (inset of Figure 1d).However, after laser scanning, the film becomes less hydro-phobic with a CA of 95.1° (left inset of Figure 1e) and goodadhesion to the substrate (right inset of Figure 1e). The contactangle hysteresis for both chemically etched and laser-texturedpatterns also reflects the different wetting properties (seeTables S1 and S2, Supporting Information). This is because thelaser beam melts the porous film and significantly reduces theroughness of the scanned areas (Figure 1e),25,26 while thesurrounding unscanned areas still keep the porosity and thesuperhydrophobicity. As a result, there is a significant differencein surface wettability at the edge of the laser-processed area(Figure 1f). Figure 1g shows a 3D surface of the stripe structurefabricated by laser direct writing. The surface roughness of thelaser-scanned area is about 60 nm lower than the surroundinguntreated area (Figure 1h). We should point out that there isstill 1H,1H,2H,2H-perfluorodecyltriethoxysilane on the surfaceinevitably after laser scanning due to a self-healing process.27−29

After laser beam scans, 1H,1H,2H,2H-perfluorodecyltriethox-

Figure 2. (a and b) Sliding angles of droplets on a patterned superhydrophobic surface with respect to droplet volume, patterning size, and titlingdirection for (a) dots and (b) lines fabricated by using LDW. (c) Top view (left), front view (middle), and right view (right) optical images of adroplet on a dot with a radius of 0.3 mm. (d) Top view (left), orthogonal direction (middle), and parallel direction (right) cross-sectional images of adroplet on a 0.8 mm wide line. (e) Optical image of the triangle arrays and illustration of directional shedding off of droplets on the triangular arraysurface. (f) Anisotropic rolling-off properties in contrasting directions for drops of different volumes. Droplet releases in direction 1 and pins indirection 2.

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ysilane reabsorbs on the laser-treated surface. This is evidencedby the XPS result showing that there is no obvious change inthe composition or chemical state of the surface before andafter laser processing. Due to the charge effect caused by thelaser beam, Ti (2p) on the surface moves to a lower energystate (458.76 eV) but is still in the form of titanium dioxide.30

O 1s exists mainly in the Ti−O bond (530.64 eV) from TiO2,31

and the Si−O bond (532.26 eV) is from fluorosilane.32 Afterlaser processing, the proportion of the Ti−O bond is reduced,which is mainly because Ti is partly sublimated in the laserprocessing. These results show that the change of CA after laserprocessing mainly comes from the change of roughness ratherthan the composition. This can also explain why the laser-scanned film has not yet become fully hydrophilic.Controllable Anisotropy of Adhesion of the Laser-

Written Surface. Patterned superhydrophobic surfaces with achanged wettability are known to provide adhesion aniso-tropy.14,33,31 The degree of anisotropy depends on the linewidth, titling direction, and surface homogeneity. Using

adhesion anisotropy, the transportation of water droplets inthe intended direction is feasible.9,34 Figure 2a and 2b showsthe effect of a laser-written dot, line size, as well as water dropvolume on the mobility of water droplets on superhydrophobicsurface. As expected, for the dot (Figure 2a), the sliding angledecreases upon increasing volume of the droplet, regardless ofthe titling direction. At a constant drop volume, the slidingangle increases upon increasing dot diameter. Lines have asimilar behavior with dots; for each line (Figure 2b), the slidingangle decreases upon increasing the volume of the droplet but,on the other hand, has a direction-controlled sliding angle, andthe droplet is easier to slide along the line than along theorthogonal direction. The strong anisotropy of the sliding angleand droplet distortion for laser-patterned surfaces is attributedto the difference in the energy barrier of wetting between thetwo directions. At smaller drop, droplets slide easier along theline than that in the vertical direction. With the increasing dropvolume, the sliding angle contrast is lower due to the largergravitational force. We can also see that for the anisotropy

Figure 3. (a) Driving tiny water droplets with controllable direction on a branch of tree-shaped hierarchical cones. (b) Optical images of hierarchicalcones fabricated on a superhydrophobic surface. (c) Schematic illustration of the method used to quantitatively measure the fog collection efficiencyof different surfaces. (d) Design of the tree pattern. Points 1, 2, and 3 represent the location where the water pipes were placed; red arrow representsthe driving direction of the water droplets. (e) Patterned film in the process of collecting water. (f) Fog-collection efficiency of different surfaces atlocations of 1, 2, and 3.

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properties in Figure 2d, compared to the dot, the line canobviously cause anisotropic movement of the droplet. Itindicates that a surface with anisotropic patterns can transportdroplets in the designed direction. We further fabricated atriangle array on superhydrophobic surfaces to test theunidirectional wetting properties, as shown in Figure 2e. Thetriangle arrays integrate a shape gradient at solid−liquidinterfaces along the tips of the triangle and thus generate therelease or the pinning of the solid−liquid contact lines. For asingle triangle, there is a maximum contact width in direction 1and a minimum contact width in direction 2; the contact width(w) can be gradiently dependent on the direction along the tipsof the triangle, causing the release of liquid in direction 1 andpinning of liquid in direction 2. These features induce a specialretention force, based on the tips of triangles. The directionalshedding-off property is examined by measuring the droprolling-off angle in the direction 1 to 2 on the patterned film.Figure 2f shows the roll-off angle of water drops with volumesfrom 3 to 15 μL. The anisotropy is enlarged with increasingdrop volume. In particular, for droplet volumes of 10−12 μL,the droplet is still pinning in direction 2 while it is very easy toroll off in direction 1. These investigations indicate that we cancontrol the mobile behavior of droplets with patterns byintegrating both the surface energy gradient and the Laplacepressure gradient, and it will be helpful for designing a noveldroplet-controlling surface that can be extended to applicationssuch as water collection, liquid transport, and cell-directedprojects.Direction-Controlled Driving of Tiny Water Drops.

LDW is an effective way to change the surface energy and tomake surface patterns. Geometry has been considered to havean important impact on the movement and the collection ofwater, and a few simple geometries have been proven toeffectively direct droplet motion.2,12,35 In the following, weshow that by taking advantage of the surface energy gradientand Laplace pressure gradient, tree-shaped hierarchical conepatterns can drive tiny drops directionally and finally collectwater in a target location. As shown in Figure 3, we first test thedirectional driving of the droplets by a branched hierarchicalcone in the horizontal plane (Figure 3a). The fog droplets areinitially captured homogeneously on the patterned surface;since there is a surface energy gradient, tiny droplets on thesuperhydrophobic region are easily pushed into the patternedregion to form larger droplets and finally gather into one bigdrop. After the droplets are collected, a new cycle of collectionbegins immediately (see Movie S1, Supporting Information). Infact, the tips of the branch generate a Laplace pressure gradientdue to the shape gradient, which further enhances thedirectional movement of water droplets.2,12,36 In more detail,because of the confined “wedge” (shape gradient), the waterdroplets cannot reach balance CAs at the gradient start becauseof the asymmetric confinement of the tip. Therefore, thedroplet has an imbalance apparent CA temporarily,14,37 i.e., anapparent CA toward the center (θinside) and an apparent CAtoward the tip (θinside′), resulting in an imbalance force. Thus, adriving force (Fwedge−wet−grad) arises from the shape gradient andwettability gradient to move the droplet directionally towardthe center of the pattern, that is

γ θ θ

θ θ

≈ − ′

+ −

− −F [(cos cos )

(cos cos )]

wedge wet grad water inside inside

inside outside (1)

,where θoutside is the apparent CA outside of the pattern. Inother words, cones have a high water collection efficiencybecause a large driving force pushes the tiny droplets towardthe collecting site. For droplets that grow up during thecollection process, when the droplet size is much larger thanthe wedge structure, as shown in Figure 3a at 12.1 s,Fwedge−wet−grad is not large enough to guide the flow of droplets;the movement of the droplets mainly depends on thecoalescence between neighboring droplets, that is, the dropletsare easily coalesced to move toward the larger cones. Asobserved in Figure 3a, the droplets finally move and spreadtoward the center of the water storage areas owing to therestriction of the cones. The surface with such cones is moreefficient in collecting water.On the basis of the above discussion, we introduce a tree-

shaped pattern with hierarchical cones for effective watercollection. The tree-shaped pattern we designed in Figure 3bconsists of a hierarchy of cone branches, ranging frommicrometer size to millimeter size. Such a size scale wellmatches that of the water drop size: a water droplet often hasan initial size of a few micrometers and grows to millimeters.The film is fixed on a sample stage in the horizontal plane andexposed under a fog flow (∼75 mg·s−1) (Figure 3c); we usepipes to collect water located at points 1, 2, and 3 as indicatedin Figure 3d. All surfaces are 10 mm × 10 mm in size. Figure 3dshows the geometry of the pattern and water collection on thefilm. The droplets were expected to be driven from branches tothe crotch and finally to the trunk and eventually flow to thespecified position 1 along the direction of the red arrow. In thecollecting process, water drops were driven along the designeddirection and the driving force causes the anisotropic shapechange of the droplet (Figure 3e). We further tested thedirectional properties of the tree-shaped film and compared theefficiency of the cases with a superhydrophobic surface and afully written film (Figure 3f). As expected, the tree-shapedhierarchical cones capture and directionally drive the majorityof water to position 1, while the other two samples do not havethe function of directional collecting water. The hierarchicalcones dramatically improved the efficiency of water collectionwith a factor of ∼36 compared to the superhydrophobicsurface, which is a quite high efficiency for water collection.While the ability to drive water droplet directionally in a

controllable manner is desired for water harvesting, it can alsobe utilized to perform interesting tasks such as “planarmicrofluidic”. Compared to the traditional microfluidictechnology, this method does not need pipelines or tunnelsbut can achieve the directional transport of liquid, similar tomicrofluidic tunnels. Meanwhile, fabrication of a 2D pattern ismore flexible and simpler than making tunnels. We can alsomanipulate tiny droplets on patterned surface with a higherresolution, such as control of the direction or amount of liquiddrops. It is interesting that the water collection process of thetree-shaped pattern is very similar to the body’s bloodcirculation system; all the water droplets captured can bedriven along the designed direction and finally flow into theroots of the tree pattern, just like the fact that blood in the bodyflows into the heart. The tree-shaped pattern is expected tosimulate the human blood system for biological research.

■ CONCLUSIONSIn conclusion, we successfully designed and fabricated tree-shaped hierarchical cones on a superhydrophobic titaniumoxide film by using LDW, which is a technique capable of

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fabricating high-resolution, large area, and complicated surfacepatterns. Such a two-dimensional pattern can precisely andcontinually drive tiny water droplets toward the root of tree-shaped patterns without the help of gravity and exhibit efficientwater collection property with an enhancement factor of ∼36compared with superhydrophobic films. The results indicatethat geometry is a key factor for the design of surface propertiesof thin films. The tree-shaped hierarchical cones might alsoopen new paths of 2D microfluidics and for simulation of thecirculation system in the human body.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.7b08116.

Contact angle hysteresis for chemically etched surface;contact angle hysteresis for laser-textured surface (PDF)Driving tiny water droplets with controllable direction ona branch of tree-shaped hierarchical cones (AVI)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: guocf@sustc.edu.cn.*E-mail: scruan@szu.edu.cn.ORCIDMeng Wang: 0000-0002-4037-4694Chuan Fei Guo: 0000-0003-4513-3117Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.FundingThe work performed at SUSTC was supported by the fundingof the National Natural Science Foundation of China (No.U1613204), the Program for Guangdong Introducing In-nnovative and Entrepreneurial Team (No. 2016ZT06G587),and “The Recruitment Program of Global Youth Experts ofChina” (No. K16251101). The work performed at NCNST wassupported by the National Key Research and DevelopmentProgram of China (2016YFA0200403), CAS Strategy PilotProgram (XDA 09020300), and National Natural ScienceFoundation of China (10974037, 61505038). The workperformed at Shenzhen University was supported by theShenzhen Science and Technology Planning (No.JCYJ20160422142912923). This work was partly supportedby the National Nature Science Foundation of China(No.61405223).NotesThe authors declare no competing financial interest.

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ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.7b08116ACS Appl. Mater. Interfaces 2017, 9, 29248−29254

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